This application claims the priority of German Application No. 197 26 539.1, filed Jun. 23, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method and a circuit for locating a short circuit or cable break in a bus system, with the short circuit or cable break in the bus system being pinpointed. The bus system is subjected at at least one point to a pulse with a steepness such that a reflection occurs at the point of the short circuit and/or cable break. The short circuit or cable break is pinpointed on the basis of the propagation time of the pulse.
In addition, a method is already known by which, under certain circumstances, a short circuit of one or both leads of a two-wire bus system to ground, or to the on-board system potential, can be detected. Also, a cable break in one or both leads can be detected (see German Patent document DE 42 12 742 A1). In this method, the voltage level on the two bus wires is compared with certain threshold values. As a function of various defects, various pairs of voltage levels are produced that are supposed to occur on the two bus wires. Therefore, the nature of the defect can be determined from an evaluation of the voltage levels.
According to the invention, it is known from DE-Z: ntz, Vol. 49 (1994), Volume 4, pp. 242-246 to determine the length of a lead and/or to locate a defect in a bus system by measuring the propagation time of a reflected signal injected at one end of the lead of the bus system.
The same is also known from German Patent document DE 37 12 780 A1, in which the propagation time is measured by a pulse being emitted again at a certain time following the reception of the reflected signal. The frequency of the pulses is measured and the propagation time can be determined from this. Pulses with alternating signs are produced.
From U.S. Pat. No. 4,151,459, in conjunction with a search for defects in a ski lift cable, it is known to measure the propagation time of a signal that is injected into the ski lift cable. The propagation time is measured using a shift register.
The distance of the short circuit and/or of the cable break from the point where the pulse is injected into the bus system can be determined by this method. Once the system topology is known, the location of the defect can be pinpointed using this method.
On the other hand, the goal of the present invention is to improve the process of pinpointing a detected defect.
This goal is achieved according to the invention by a method in which the bus system is a two-wire bus system which is subjected to a positive pulse when a short circuit to ground is detected and is subjected to a negative pulse when a short circuit to system potential is detected.
The choice of pulses determines the reflection of the pulse that is necessary for evaluation. From the previous error analysis it is known whether a short circuit to ground or to system potential is involved, so that the sign of the pulse can be selected as a function of this evaluation. The previous error analysis can proceed for example by the method described at the outset or by the method described in the application of Applicant on the same application date that is pending under File Number 197 26 538.3 at the German Patent Office (corresponding to U.S. Ser. No. 09/102,629, filed on even date herewith and entitled "Method and Circuit for Checking Lead Defects in a Two-Wire Bus System", the specification of which is expressly incorporated by reference herein).
In the method according to the invention, the pulse is at least approximately a pulse function.
As a result, first of all, a sufficiently high slope of the pulse is achieved so that a reflection occurs. On the other hand, assurance is provided that the energy content of the pulse is limited to a finite value, so that errors caused by checking can be avoided.
In the circuit according to the present invention for performing one of the above methods, a pulse generator is connected with a chain of logic gates having a shift register, with the passage of the signal through the gate chain being started when a pulse is emitted into the two-wire bus system, whereupon the reflected signal interrupts the passage of the signal through the gate chain, so that the distance of the short circuit or cable break from the point where the pulse generator is connected to the two-wire bus system can be determined from the number of gates that have been traversed, with the pulse generator transmitting a positive pulse when a short circuit to ground is detected, and a negative pulse when a short circuit to system potential is detected.
The method according to the invention can be performed comparatively simply with this circuit.
In a method for locating a cable break according to the invention, a cable break is determined to have taken place in the connecting leads of a subscriber when this subscriber cannot be reached by communication over the bus.
As a result, this defect can also be detected, while otherwise this defect cannot be detected by continuous communication.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.